It would require volumes to trace the history of the growth of these early mammals, and show how they, step by step, came to their present higher state. We will take only one of the simplest of these changes, which happens to be also the one which we know best. This is the change that led to the making of our common horses, which seem to have been brought into life on the continent of North America. The most singular thing about our horses is that the feet have but one large toe or finger, the hoof, the hard covering of which is the nail of that extremity. Now it seems hard to turn the weak, five-fingered feet of the animals of the lower Tertiary--feet which seem to be better fitted for tree-climbing than anything else--into feet such as we find in the horse. Yet the change is brought about by easy stages that lead the successive creatures from the weak and loose-jointed foot of the ancient forms to the solid, single-fingered horse's hoof, which is wonderfully well-fitted for carrying a large beast at a swift speed, and is so strong a weapon of defence that an active donkey can kill a lion with a well-delivered kick. [Illustration: FIG. 10. FEET OF TERTIARY MAMMALS.] The oldest of these creatures that lead to the horses is called _Eohippus_ or beginning horse. This fellow had on the forefeet four large toes, each with a small hoof and fifth imperfect one, which answered to the thumb. The hind feet had gone further in the change, for they each had but three toes, each with hoofs, the middle-toed hoof larger and longer than the others. A little later toward our day we find another advance in the _Orohippus_, when the little imperfect thumb has disappeared, and there are only four toes on the forefeet and three on the hind. Yet later we have the _Mesohippus_ or half-way horse. There are still three toes on the hind foot, but one more of the fingers of the forefeet has disappeared. This time it is the little finger that goes, leaving only a small bone to show that its going was by a slow shrinking. The creature now has three little hoofs on each of its feet. Still nearer our own time comes the _Miohippus_, which shows the two side hoofs on each foot shrinking up so that they do not touch the ground, but they still bear little hoofs. Lastly, about the time of man's coming on the earth, appears his faithful servant, the horse, in which those little side hoofs have disappeared, leaving only two little "splint" bones to mark the place where these side hoofs belong. Thus, step by step, our horses' feet were built up; while these parts were changing, the other parts of the animals were also slowly altering. They were at first smaller than our horses,--some of them not as large as an ordinary Newfoundland dog; others as small as foxes. [Illustration: FIG. 11. DEVELOPMENT OF HORSES'S FOOT.]